This is going to sound geeky and like it would take an immense amount of time...
Okay, it is geeky.
But it really doesn't take a lot of time.
Go to the computer reference section of a bookstore. Pickup the Excel Bible or a QUE book, or whatever on the latest edition that has all the functions and just flip through each page.
I do this everytime a new version comes out. It really doesn't take a lot of time if you know a majority of the functions (takes me about 20 minutes). Also refreshes your awareness of functions that you forget because you don't use them regularly.
Much better than some stupid course.